According to the ENERGY HANDBOOK published by Van Nordstrom and Reinhold, there are 36.5 Quads ( one Quad=10.sup.18 Btu) of energy in the coal reserves of the United States as compared to 1.4 Quads in oil and 111 Quads in natural gas. The United States Department of Energy (DOE) has taken the lead in the development of methods to utilize this energy in coal in an ecologically and economically acceptable manner. The DOE has taken the initiative in this matter because industry in the United States has been reluctant to pursue this task, and an agency of the U.S. Government has been forced to take the lead just as was the case in the development of synthetic rubber and nuclear power.
There is a further necessity to develop methods for the utilization of coal because of the political instability in the Persian Gulf which is a major source of much of the world's oil, and the wide fluctuations in the price of crude oil in the last few years are clearly indicative of this instability.
Evidence continues to mount with regard to the detrimental effects of acid rain. Most of the effort to reduce the emission of sulfur from presently operating power plants using steam turbines for the generation of electricity have been concentrated on the removal of sulfur dioxide from the flue gases after the complete combustion of the coal which requires the installation of scrubbers between the boiler of such power plans and the smoke stack.
The government of Canada has obtained an agreement with the United States for reduction of sulfur oxides (SO.sub.x) and nitrogen oxides (NO.sub.x) in flue gases from coal burning power plants in the United States. These power plants consume most of the coal mined in the U.S. There is a further agreement that the reduction in SO.sub.x and NO.sub.x will occur when effective and economical technology has been developed for their removal from flue gases. The U.S. Government and the power-generating industry in the U.S. are spending approximately one billion dollars in the next few years in search of this effective and economical technology for removal of SO.sub.x and NO.sub.x, but to date, no completely satisfactory solution has been found. At present, most of this work is concentrated on achieving present Environment Protection Agency (EPA) regulations which require that the effluent from new power plants contain less than 1.2 lbs. SO.sub.2/ MMBtu of the fuel consumed. Some of the latest information on this subject was reported at the Fourth Annual Pittsburgh Coal Conference held in 1987.
Almost all of this money that will be spent over the next few years will be spent on pilot scale and demonstration units in an attempt to find a method of using calcium-based sorbents that will reduce SO.sub.2 from flue gases to meet EPA requirements. The ability of these calcium-based sorbents will be investigated at both high (800.degree. C.-1200.degree. C.) and low (100.degree. C.-150.degree. C.) temperatures. The high temperature processes include the Limestone Injection Multiple Burner (LIMB) method, the pressurized fluid-bed combustion (PFBC) technique, and the atmospheric pressure fluid-bed combustion (AFBC) method. The low temperature process to be investigated is a variation of "flue gas scrubbers" or scrubbers which are currently in operation in 140 power plants in the U.S.
There are many shortcomings to these methods: (1) the expected desulfurization of all these processes is less than 50% of the input sulfur except for the fluid bed combustion processes where the sulfur reduction may be as high as 70%; (2) indications are that there will be less than 50% utilization of the calcium-oxide based sorbent; (3) the fundamentals of desulfurization of flue gases with calcium based sorbents are not well understood which may result in less than optimum desulfurization of the gases and utilization of the sorbent; (4) control of temperature in the low temperature desulfurization process must be held to plus or minus 20.degree. F. which may be difficult when a boiler is operating in a load following mode; (5) with the low temperature processes the calcium based sorbents are introduced into the flue gases with water before the electrostatic precipitators (ESP) which reduces the temperature of the flue gases so that the stacks do not operate effectively; (6) there is no certainty that it will be possible to put the partially sulfated calcium sorbent into landfills because of: ( a ) potential environmental effects associated with handling fine caustic materials, (b) the heat generated by the reaction of unreacted lime with atmospheric moisture or water, (c) the corrosive nature of very basic solutions, (d) the increased quantities of solid waste. In addition, critical pieces of the equipment necessary to utilize calcium based sorbents are not presently available particularly with respect to the AFBC.
In addition to all of the operating and ecological problems mentioned above, it has been estimated that it costs approximately $900 to remove a ton of sulfur with scrubbers and $600 to $800 to remove a ton of sulfur with the LIMB process. The costs of SO.sub.x removal from flue gases by these methods will raise the cost of electricity by as much as one third as compared to the generation of electricity without sulfur removal.
The effluent from coal fired power plants is one of the major sources of SO.sub.2, but there are other significant sources of SO.sub.2 such as the effluent from roasters utilized to convert the ores of metals such as copper and nickel from some form of sulfides or sulfates to their oxides with the emission of large quantities of SO.sub.2. The roasters of the International Nickel Co. in Sudbury, Ont. Canada are one of the major sources of SO.sub.2 emissions in North America. The analysis of the gases resulting from the roasting of nickel or copper are not sufficiently different from the analysis of stack gases from boilers that desulfurization of roaster gas cannot be obtained with cerium oxide.
Technology for the desulfurization of fuel gases in which the sulfur is mainly in the form of hydrogen sulfide (H.sub.2 S) is presently oriented toward the use of in situ desulfurization with calcium-based sorbents in a fluid bed gasifier and a yet undeveloped sorbent for in situ desulfurization in entrained flow gasifiers. Desulfurization in situ with calcium-based sorbents in fluid bed gasifiers has achieved 90% reduction of H.sub.2 S in the effluent gases.
The use of calcium-based sorbents for in situ desulfurization in fluid bed gasifiers suffers from most of the same problems associated with calcium-based sorbents for flue gas desulfurization except 90% sulfur removal has been achieved. There is no assurance the 90% sulfur removal can be consistently achieved which is necessary to meet present EPA requirement (less than 1.2 lbs. SO.sub.2/ MMBtu) in the gases finally exiting the process.
To insure compliance with present and future requirements for SO.sub.2 emissions from processes using fuel gases and because of the uncertainty regarding disposal of partially spent calcium-based sorbents, additional effort is being expended to develop a regenerable sorbent for H.sub.2 S capable of (1) reducing the H.sub.2 S content of fuel gases to less than 100 part per million (ppm); (2) being regenerated back to its original form which is capable of again reacting with H.sub.2 S with little or no loss of its ability to remove H.sub.2 S from the fuel gases; (3) capable repeated cycles of sulfidation and regeneration.
Zinc ferrite is under exhaustive investigation to determine if it is capable of meeting these requirements. It is recognized that zinc ferrite has at least six possible deficiencies: (1) some solid phase boundaries which reduce stability and effectiveness; (2) sulfate formation during regeneration; (3) sorbent structural changes during sulfidation and regeneration cycles; (4) sorbent durability over many sulfidation and regeneration cycles; (5) regeneration of gas must be processed to remove sulfur; (6) the zinc component of the sorbent may vaporize at high operating temperatures. In addition, zinc ferrite cannot operate with fuel gases high in CO and H.sub.2 because such gases reduce the zinc ferrite to a form of zinc oxide and iron oxide or their metals that are less capable of reacting with the sulfur.
Almost 300 million dollars has been accumulated by pooling funding from the DOE and industry to build a demonstration unit to confirm the applicability of in situ desulfurization of fuel gases with zinc ferrite polishing to produce fuel suitable for gas turbines for the next generation of electric power generating equipment.
Cerium oxide also has the ability to desulfurize fuel gases which are high in carbon monoxide and hydrogen where the sulfur is mainly in the form of H.sub.2 S, and flue gases which are high in carbon dioxide and oxygen where the sulfur is mainly in the form of SO.sub.2. This unique ability to desulfurize gases of such wide variations in composition may be explained with the use of the Ce-S-O phase stability diagram, FIG. 1. The ordinate of this phase stability diagram is the logarithm of the partial pressure of oxygen (Log pO.sub.2) and the abscissa is the log of the partial pressure of sulfur (Log pS.sub.2). Fuel gases would be composed mainly of carbon monoxide, hydrogen, carbon dioxide, water and hydrogen sulfide. The oxygen content in such gases is very small and results from the dissociation of water and carbon dioxide at elevated temperatures. The partial pressure of sulfur in such gases results from the dissociation of the hydrogen sulfide. As a result, the ability of cerium oxide to desulfurize such gases is related to the sum of the partial pressure of oxygen from dissociation of the water and the carbon dioxide in the fuel gas.
Other gases such as those created by the distillation of crude oil necessary for the production of gasoline, motor oil etc. contain sulfur in the form of H.sub.2 S or sulfur carbonyl (COS). These gases may contain in addition to CO and H.sub.2 other hydrocarbons, but the partial pressure of oxygen in such gases is low, and they can be effectively desulfurized with cerium oxide as well as fuel gases derived from coal.
When considering flue gases, the partial pressure of the oxygen is mainly related to the amount of oxygen in the gases and the oxygen resulting from the dissociation of any carbon dioxide in the system is a minor component of the total. The partial pressure of sulfur in flue gases results from the dissociation of the sulfur dioxide. As a point of reference the partial pressure of oxygen in air is noted on the phase stability diagram (pO.sub.2 =0.21).
The limits of desulfurization with cerium are defined by the line labeled XYZ. Point B on that line is a value calculated from thermodynamic data representing a fuel gas containing 240 ppm H.sub.2 S which serves as a point of reference for the extent of desulfurization with cerium oxide.
There are two forms of cerium oxide: ceric oxide (CeO.sub.2) and cerous oxide (Ce.sub.2 O.sub.3 or CeO.sub.1.5) and there are a number of nonstoichiometric forms of cerium oxide whose composition may be expressed generically as CeO.sub.(2-x).
The extent of desulfurization of flue gases may be estimated from the phase stability diagram as being determined from the intersection of the line representing the partial pressure of oxygen in air and line XYZ. Since point B represent 240 ppm of H.sub.2 S in a fuel gas, the intersection of the line representing the partial pressure of oxygen in air and line XYZ is several orders of magnitude lower in sulfur.
The Ce-O-S system encompasses the desulfurization of gases containing both H.sub.2 S and SO.sub.2. The thermodynamic principals and the benefits of using solid solutions of cerium oxide which crystallizes in the fluorite habit apply equally well to either type of gaseous desulfurization.
Bevan and Kordis (MIXED OXIDES OF THE TYPE MO.sub.2 (FLUORITE)-M.sub.2 O.sub.3 -I: OXYGEN DISSOCIATION PRESSURES AND PHASE RELATIONSHIPS IN THE SYSTEM CeO.sub.2 -Ce.sub.2 O.sub.3 AT HIGH TEMPERATURES; Jr. Inorganic Nuclear Chem., 1964, Vol. 26, pp. 1509-1523, Permagon Press, Ireland.) have stated: "A characteristic property of MX.sub.2 compounds crystallising with the fluorite structure [habit] is the readiness with which the cation lattice can incorporate quite a large proportion of altervalent ions to form `anomalous mixed crystals` [solid solutions] for which the fluorite structure is apparently retained. Evidence that the cation lattice is virtually complete and the anion lattice highly defective [creation of oxygen ion vacancies] has been obtained from a comparison of X-ray and pycnometric densities, so that depending on the nature of the altervalent ion, the mixed crystal is thought to contain either vacancies or interstitial anions. Moreover, the distribution of altervalent cations on cation sites, and of the anion defects is generally assumed to be completely random."
The Bevan and Kordis article provides the scientific explanation for a "solid solution" as follows: "Evidence that the cation lattice is virtually complete . . . has been obtained from a comparison of x-ray and pyconometric densities . . . . Moreover, the distribution of altervalent cations on cation sites . . . is generally assumed to be completely random."
The Bevan and Kordis article does not address how the oxygen ion vacancies created could be utilized. Therefore, this article does not teach or suggest the ability of solid solutions of cerium oxide which crystallize in the fluorite habit containing oxygen ion vacancies to increase the extent of desulfurization, the utilization of the sorbent and the rate of desulfurization.
The use of cerium oxide for reaction with sulfur dioxide (SO.sub.2) is described in Longo, U.S. Pat. No. 4,001,375 and Cahn, U.S. Pat. No. 4,346,063. Cahn teaches a technology whereby gases containing SO.sub.2 and oxygen in amounts sufficient to prevent the formation of hydrogen sulfide (H.sub.2 S) are desulfurized with cerium oxide. Cahn further states that the temperature of desulfurization is from 350.degree. C.-600.degree. C., but Longo states that the rate of desulfurization of flue gases containing SO.sub.2 with CeO.sub.2 is low until the temperature of the reaction reaches 500.degree. C. Because of the high rate of speed with which the products of combustion proceed through a boiler and because of the fluctuations in temperature when the boiler operates at various loads due to variations in demand for electricity it is required that a rapid rate of desulfurization be obtained at temperatures less than 500.degree. C.
In addition to the necessity to increase the rate of reaction, it is also necessary to increase the utilization of the sorbent. Longo has achieved 50-70% utilization of an unsupported sorbent, but in order to reduce the cost of flue gas desulfurization with cerium oxide (CeO.sub.2), utilization of the sorbent should be increased. Evidence will be supplied in the Examples to follow which shows that the rates of reaction of doped CeO.sub.2 is greater than that of undoped CeO.sub.2.
The use of CeO.sub.2 for the desulfurization of fuel gases has been described by Wheelock et al., U.S. Pat. Nos. 4,002,720 and 3,974,256. However, neither Wheelock et al. reference teaches or suggests:
1. Difference between those lanthanides which crystallize in the fluorite habit and those that crystallize in other habits on their ability to desulfurize fuel and flue gases; PA1 2. The importance of oxygen ion vacancies to enhance the ability of the lanthanides which crystallize in the fluorite habit to desulfurize both fuel and flue gases; PA1 1. Gaffney, U.S. Pat. No. 4,560,823; PA1 2. Compton et al., U.S. Pat. No. 3,892,836; PA1 3. Mulhlhaupt, U.S. Pat. No. 3,980,763; PA1 4. Takada et al., U.S. Pat. No. 4,849,398; PA1 5. Enomoto et al., U.S. Pat. No. 3,901,947; PA1 6. Ryland et al., U.S. Pat. No. 3,277,184; PA1 7. Bauer et al., U.S. Pat. No. 3,425,793; PA1 8. Mester et al., U.S. Pat. No. 4,642,177; and PA1 9. Sergeys et al., U.S. Pat. No. 3,903,020. PA1 1. That those lanthanides which crystallize in the fluorite habit are best suited for the desulfurization of gases; PA1 2. That solid solutions of lanthanide oxides which crystallize in the fluorite habit and other altervalent oxides of alkaline earth elements or lanthanide oxides which do not crystallize in the fluorite habit or combinations of the other oxides have improved ability to desulfurize gases compared to lanthanides which crystallize in the fluorite habit without the addition of other oxides; PA1 3. The function of oxygen ion vacancies created by reduction and the addition of altervalent oxides to the lanthanides which crystallize in the fluorite habit to enhance the removal sulfur from both fuel and flue gases; and PA1 4. The limitations of the amount of the addition of altervalent oxides of the alkaline earth elements or the other oxides of the lanthanide group which are altervalent to CeO.sub.2 and do not crystallize in the fluorite habit which together with the CeO.sub.2 form the solid solutions of this invention.
3. That the formation of solid solutions of those lanthanides which crystallize in the fluorite habit and the oxides of the alkaline earth elements or lanthanide oxides which do not crystallize in the fluorite habit can create additional oxygen ion vacancies in lanthanides which do crystallize in the fluorite habit; and
4. That the optimum amount of other oxides added to the lanthanide oxides which crystallize in the fluorite habit may be less than 15 mole %.
Various other patents which utilize lanthanides have been found which include:
However, none of these patents teach or suggest:
Since the basis of this application is the creation and the utilization of "oxygen ion vacancies" in the fluorite type crystal lattice of the lanthanide oxides, it is necessary to describe one of the methods whereby the "oxygen ion vacancies" are formed. For this explanation cerium oxide, one of the lanthanide oxides which crystallizes in the fluorite habit, and magnesium oxide will be used, and the same combination of cerium and magnesium oxides may be used hereafter to typify this phenomenon. The use of oxides of cerium, and magnesium in any of the illustrations or examples in no way precludes the use of any of the oxides of the other members of the lanthanide group of elements that crystallize in the fluorite habit, other than cerium, nor does the use of the oxide of magnesium preclude the use of the oxides of any of the other members of the alkaline earth group of elements unless it is specifically noted as to the use of the specific members of the group of lanthanides or the group of alkaline earth elements.
The mechanism whereby the oxygen ion vacancies are formed in these solid solutions which crystallize in the fluorite habit may be explained as follows. In the discussion which follows, the cerium oxide of the solid solution which is formed will be the solvent, and the solute will be an oxide or combinations of one or more of oxides of the alkaline metals group or the oxides of other lanthanide which do not crystallize in the fluorite habit and whose valence is different (altervalent) than that of CeO.sub.2. When one of the solute oxides of the alkaline earth elements such as MgO is in solution in the solvent cerium oxide crystal, the cation (Mg.sup.+2) substitutes for one of the cerium cations (Ce.sup.+3) or (Ce.sup.+4). Since there can be no imbalance in electrical charges, the substitution of one Mg.sup.+2 ion for one Ce.sup.+4 or two Ce.sup.+3 ions, creates one O.sup.-2 vacancy in the lattice. This is exactly in accordance with the mechanisms described by Bevan and Kordis.
The general chemical formula for substances which crystallize in the fluorite habit is MX.sub.2 where M represents one metal cation and X.sub.2 represents two oxygen anions.
When altervalent solute oxides are added to solvent oxides such as CeO.sub.2, the CeO.sub.2 is said to be "doped." Hereinafter the term "doped CeO.sub.2 will refer to solid solutions containing oxygen ion vacancies in which the CeO.sub.2 is the solvent and the altervalent oxides added to form the solid solutions which crystallize in the fluorite habit is the solute which is called the "dopant."
Again using cerium oxide and magnesium oxide for illustration purposes, the cerium oxide without the magnesium oxide is like a checkerboard with a checker in each square, which immobilizes all the checkers except those at the edge. However an oxygen ion vacancy in the crystal lattice of the cerium oxide would be analogous to removing at least one checker from the checkerboard whereby all of the other checkers become more mobile. If the goal was to replace the checker in the exact center of the board, it would be much simpler if there were many vacancies on the board so that the new checker could be maneuvered into the center by moving the other checkers to make multiple paths to the center of the checkerboard.
All dopants that have the same valence that form solid solutions with lanthanide oxides that crystallize in the fluorite habit form equal numbers of oxygen ion vacancies, and they will be referred to hereinafter as "doped oxygen ion vacancies." The effectiveness of the dopant with regard to the conductivity in solid electrolytes is a maximum when the ionic radius of the dopant is equal to the ionic radius of the lanthanide being doped. If the ionic radium of the dopant is different than the ionic radius of the lanthanide oxide being doped, there will be distortion of the crystal lattice, (the binding energy between the dopant cations and the oxygen ion vacancies will increase) and the ionic conductivity of the lanthanide oxide will be reduced. It is expected that the resistance to the entry of sulfur into the doped lanthanide crystals and the escape of oxygen from the crystals will increase as distortion of the crystal lattice increases.
The reaction for the removal of sulfur as H.sub.2 S from gases resulting from the partial combustion of coal (fuel gases) can be simply expressed as follows: EQU 2CeO.sub.2 (s)+H.sub.2 S(g)+H.sub.2 (g)=Ce.sub.2 O.sub.2 S(s)+2H.sub.2 O(g) (1)
In the reaction described in equation (1) there are two kinds of checkers (anions) that are moving in the crystal lattice. The anions of sulfur (S.sup.-2) are moving into the lattice of the cerous oxide and anions of oxygen (O.sup.-2) are moving out. The ultimate goal of all desulfurization processes is to use as much of the desulfurization agent as possible. That goal can best be achieved by increasing the ease of movement of the sulfur anions trying to get into the cerium oxide crystal lattice and by increasing the ease of movement of the oxygen anions out of the crystal lattice.
The most common form of cerium oxide is ceric oxide (CeO.sub.2). The best form of cerium oxide for removal of H.sub.2 S is cerous oxide (Ce.sub.2 O.sub.3). There is, however, a series of non-stoichiometric forms of cerium oxide between ceric oxide and cerous oxide. These non-stoichiometric forms of cerium oxide can be prepared by exposing ceric oxide to reducing gases such as hydrogen. When ceric oxide is converted to these non-stoichiometric forms, whose formula can be written as CeO.sub.2-x there is a loss of weight due to removal of oxygen from the crystal lattice (formation of oxygen ion vacancies). Oxygen ion vacancies created by exposure of cerium oxide to reducing gases will hereinafter be referred to as "reduction oxygen ion vacancies". Cerium oxide is no longer in the fluorite crystal habit when reduction of cerium oxide to an oxidation state lower than CeO.sub.1.714 is achieved, and this is likely to result in the elimination of the reduction oxygen ion vacancies. FIG. 2, from Bevan and Kordis, shows the extent of removal of oxygen from CeO.sub.2 with reducing gases whose reducing power is measured by their partial pressure of oxygen (pO.sub.2). The equation for the removal of O.sub.2 from CeO.sub.2 may be written as follows: EQU CeO.sub.2 (s)+XH.sub.2 (g)=2CeO.sub.( 2-x)(s)+H.sub.2 O(g) (2)
The effect of oxygen ion vacancies created both by reduction and doping have a cumulative effect with respect to improving the ability of CeO.sub.2 to remove sulfur from fuel gases. between that of ceric and cerous oxide such as CEO.sub.1.92. Based on this loss of oxygen and Avogadros number, it can be calculated that there are 2.75.times.10.sup.20 oxygen vacancies per gram of CeO.sub.1.92.
However, for the removal of sulfur from flue gases created by the complete combustion of sulfur containing hydrocarbons, which may contain 3 or 4% oxygen, no reduction oxygen ion vacancies are formed.